Mini Split Sizing

[magic_garage]

I'm from Hawaii and I'm having trouble understanding sizing a AC unit but I'm looking at some sizing charts, forums and what I hear from friends and family. Bottom line, it really seems like everyone from my state sizes their units a lot bigger than what I'm gathering on the internet. For example, I used a calculator from a site and attached is the real rough info I entered. I realize after that the window information is really off (way too big) but more my point. The results came up with a 11,600 BTU/hr load so 12K size unit(?). I will go out on a limb and say no one in my state will get a size that small for the size of my living room. I had a ctr take a look, and he suggested 18K but he didn't seem to look and measure real good. My aunt just got her living room unit replaced and we have a significantly bigger area, and their ctr got them a 24k unit. My co-workers living room may be roughly close in size to mine, I think I edge him out a little, and their ctr got them a 32 or 36K unit (i forget). Are all these people always running their units on the lowest fan setting and does that make a difference in terms of efficiency? I know I'm not providing better specifics but there just seems to be a way huge delta when I'm asking the circle of people around me about their AC units. I think this matters and I will say, in Hawaii, most builders are not concerned or really care much or just have the knowledge about insulating homes "well". I think our climate is so consistent the builders can do a sloppy job and get away with it. So what gives here?

Side Info: It's 11pm and it's 77 degrees at 66% humidity in my living room now, and it's def a cooler time of the year now.

 

[magic_garage]

Ok, so I got another ctr to provide me with an estimate. He said min 24k but I should really consider 30k. Again, why is there such a huge delta? Anyone?

I was chatting with my friend and he has a 30k unit for his living room. I don't know his living room SF but it's much different than mine. I will bet anything it is way over the "charts" recommendations. He himself thinks its oversized. He said he usually has his unit set to 77 F, and 1 maybe 2 tics of the fan setting.

Mid-day the other day, he said his living room measured 73 F at 55% humidity (outside around high 60's %). The unit was set to 77 F. I don't know how well his unit is maintained so I know those data points could be real off but it was something.

 

[magic_garage]

Ok, so I got another ctr to provide me with an estimate. He said min 24k but I should really consider 30k. Again, why is there such a huge delta? Anyone?

I was chatting with my friend and he has a 30k unit for his living room. I don't know his living room SF but it's much different than mine. I will bet anything it is way over the "charts" recommendations. He himself thinks its oversized. He said he usually has his unit set to 77 F, and 1 maybe 2 tics of the fan setting.

Mid-day the other day, he said his living room measured 73 F at 55% humidity (outside around high 60's %). The unit was set to 77 F. I don't know how well his unit is maintained so I know those data points could be real off but it was something.

Meant to say my friends living room SF is NOT MUCH different than mine.

 

[fitter30]

A oversize unit can't pull humidity out of the space. Air temp is easy to lower but RH - latent heat is the hard part. How many times have u walked into a space felt cool and 5 minutes later not so much. Cooled the air not brought down the RH. Undersize can catch up at the extreme temp. If design temp is 95° @ 76° and it gets to 100°
76° turns to 81° running the equipment 24/7 hopefully people don't complain till late afternoon a few days a years.

 

[jjrbus]

Almost every house I have been in here in Florida the AC is oversized as far too many of the AC companys are bozo's. When the professional guesstimaters guess, they invariably oversize for fear of not being big enough. There is no more guessing on AC there is a manual J. Shocking how many AC people do not do a manual J. Also most inverter Mini modulate, check with manufacturer, my 9000 btu Daikin can change output from 2000 to 11000 btu! Have you had a window AC in this room, did it work? A mini is more efficient when it is working hard and also removing more humidity. I live in a swamp so this I can verify.

In general mini split thermostats are not accurate, the sensing unit is in the head unit on the wall not where you are sitting. I am not an AC expert, so my opinion is worth what you are paying for it.

https://www.loadcalc.net/

 

[WildBill]

My understanding is because mini-split systems use a variable power inverter compressor system they are fine when oversized. Unlike older systems that just cycle off/on at full power. And because it's so humid in Hawaii having the larger capacity helps with removing moisture. But I am not an expert, that's just what I've read on the interwebs.

 

[Jackfre]

You don’t give any dimensions for the space. Mini-splits in my experience tend to exceed yoour expectations. I would go with what the sizing chart recommends giving consideration to orientation of windows. Do you have large W-NW facing windows that will tend to burn you down on a summer day? I guess the compromise from the contractor to the sizing chart would be a 15k unit. As pointed out they do modulate so compare the low speeds outputs. When you first start a mini up, in a humid environment it will go to low speed and wring the moisture out of the air. As noted, contractors always oversize. They don’t know how the building is insulated, etc and they don’t want a call. Also, they are used to mostly dealing with single stage equipment. There is a world of difference single vs modulating and you need to live with it to appreciate it.

 

[magic_garage]

Almost every house I have been in here in Florida the AC is oversized as far too many of the AC companys are bozo's. When the professional guesstimaters guess, they invariably oversize for fear of not being big enough. There is no more guessing on AC there is a manual J. Shocking how many AC people do not do a manual J. Also most inverter Mini modulate, check with manufacturer, my 9000 btu Daikin can change output from 2000 to 11000 btu! Have you had a window AC in this room, did it work? A mini is more efficient when it is working hard and also removing more humidity. I live in a swamp so this I can verify.

In general mini split thermostats are not accurate, the sensing unit is in the head unit on the wall not where you are sitting. I am not an AC expert, so my opinion is worth what you are paying for it.

https://www.loadcalc.net/

Never ran a AC in this living room. In my town, you didn't "need" AC, maybe a couple bad days throughout the year you wish you had one, but in the last few years, it's just gotten hotter so now there are too many bad days to try and tolerate.

I visited the website and tried to enter my data as best I could and here's the results. It says min 18.5K and max 22K so does that mean I get the 18k or 24k? I'm going for a Mitsubishi so those are my options.

 

[magic_garage]

You don’t give any dimensions for the space. Mini-splits in my experience tend to exceed yoour expectations. I would go with what the sizing chart recommends giving consideration to orientation of windows. Do you have large W-NW facing windows that will tend to burn you down on a summer day? I guess the compromise from the contractor to the sizing chart would be a 15k unit. As pointed out they do modulate so compare the low speeds outputs. When you first start a mini up, in a humid environment it will go to low speed and wring the moisture out of the air. As noted, contractors always oversize. They don’t know how the building is insulated, etc and they don’t want a call. Also, they are used to mostly dealing with single stage equipment. There is a world of difference single vs modulating and you need to live with it to appreciate it.

My SF came out to 727 sf.

 

[magic_garage]

I really appreciate all who responded.

So with the inverter type mini splits, which the motor vary, then even if the unit is oversized, it shouldn't matter since it's running low vs, cycyling off like the older ones? So aside from throwing extra dollars at the oversized unit up front, what are the pitfalls? Will the humidity issue still be in play here even though the unit slows down? This part is still confusing to me.

 

[jjrbus]

Never ran a AC in this living room. In my town, you didn't "need" AC, maybe a couple bad days throughout the year you wish you had one, but in the last few years, it's just gotten hotter so now there are too many bad days to try and tolerate.

I visited the website and tried to enter my data as best I could and here's the results. It says min 18.5K and max 22K so does that mean I get the 18k or 24k? I'm going for a Mitsubishi so those are my options.

I am not an AC pro! I would prefer to err on the low side. Bigger equipment costs more, costs more to operate and does not dehumidify well. I know nothing about Mitsubishi but I would guess if you go poke around their website you will find an 18K may go as low as 10K and may go as high as 22K. The harder it is working the more efficient it is and the more humidity will be removed. I have a 12K Daikin in my livingroom which called for 11.5K, except for the hottest of days in summer the unit loafs and never runs all out. If I replace it I will put in a 9K!!

There are some knowledgeable people at this forum ask them https://www.diychatroom.com/forums/hvac.17/

 

[magic_garage]

Hi. Just wanted provide an update on my decision. I went with a 18K Mitsubishi unit. It cools just fine but this is also the coolest time of the year of us so hard to say what will happen during the summer. I haven't been running it a whole lot but it'll lower the humidity about 15% based on a thermometer I been using.

I appreciate all the response.


Thanks!